The present invention relates to a uniformity correction apparatus for gradation and focus for reducing local unevenness in brightness, color and focus in a display screen to be able to produce a beautiful and faithful image.
FIG. 1 is a schematic diagram illustrating a typical CRT display in a prior art. In FIG. 1, numeral 1 denotes an input image signal C, 2 a gain control circuit, 3 a gain control terminal, 4 an output amplifier, 5 a CRT, 3" a focus control signal input terminal, and 3' a focus control amplifier.
Recently, the degree of the uniformity required for the display screen increases rapidly as application fields of an image display are spread. Thus, a digital circuit technique is utilized to try to improve a screen picture.
An example of a conventional technique is shown in FIG. 2. In FIG. 2, numeral 6 denotes a horizontal synchronizing signal input terminal, 7 a phase detector, 8 a voltage controlled oscillator and 9 a counter. The phase detector 7, the voltage controlled oscillator 8 and the counter 9 constitute a known PLL circuit. The counter 9 produces a 6-bit horizontal position signal Xo.
Numeral 10 denotes a vertical synchronizing signal input terminal and 11 a counter for counting the number of scanning lines and which is reset by a vertical synchronizing signal. The counter produces a vertical position signal Yo. When the number of scanning lines of the input signal is 1000, the bit number of Yo is 10 bits.
Numeral 12 denotes an E.sup.2 PROM (64K bytes) for storing data for uniformity correction of a screen. The E.sup.2 PROM is supplied with the horizontal and vertical position signals Xo and Yo as its input address signals and produces the correction data. Numeral 13 denotes a D/A converter which produces an analog signal. Numeral 14 denotes an output terminal.
When the uniformity of gradation is to be corrected, the output terminal 14 of FIG. 2 is connected to the terminal 3 of FIG. 1.
When the uniformity of focus is to be corrected, the terminal 14 is connected to the terminal 3".
When the uniformities for gradation and focus are to be corrected simultaneously, two circuits shown in FIG. 2 are required therefor.
As U.S. patents relevant to the present invention, there are U.S. Pat. Nos. 5,194,783 and 4,980,614. Moreover, a single scan display is disclosed in "Large-Screen HDTV Monitor Development" by L. Thorpe et al, August 1990, pp 620.about.633.
The prior art technique disclosed in the U.S. patents performs satisfactorily in a dedicated display unit for displaying a signal in a single scanning format.
However, when it is applied to a so-called auto-scan type display unit capable of treating a plurality of scanning formats, the prior art technique is unsatisfactory because the address signals Xo and Yo do not have one-to-one relation to position coordinates on the screen.
Accordingly, heretofore, there is provided a dedicated E.sup.2 PROM's 12 for each scanning format, and when the scanning format for the input signal is changed over, the E.sup.2 PROM's 12 are required to be switched in synchronism with the changing-over of the scanning format.
In other words, heretofore, it is necessary to use many expensive E.sup.2 PROM's in accordance with the scanning formats.
It is also necessary to switch the E.sup.2 PROM's in accordance with the scanning format of the input signal.